2 pencils cost $3.52. Which equation would help determine the cost of 13 pencils?
There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 13 pencils. Since 13 pencils cost $x$ , we have the following proportion: $\dfrac{13}{x}$ We can write the fact that 2 pencils cost $3.52 as a proportion: $\dfrac{2}{\$3.52}$ The cost changes along with the number of pencils purchased, and so the two proportions are equivalent. $\dfrac{13}{x} = \dfrac{2}{\$3.52}$